1. Field of the Invention
The invention relates to a continuous process for preparing alkylchlorosilanes, in which the residues of direct synthesis of alkylchlorosilanes are cleaved thermally using hydrogen chloride.
2. Background Art
During the direct synthesis of alkylchlorosilanes of the general formula RaHbSiCl4-a b, in which a is 1, 2, 3 or 4 and b is 0, 1 or 2, from metallic silicon and alkyl chlorides R—Cl, where R is an alkyl radical, oligosilanes, carbosilanes, siloxanes and high-boiling cracking products are produced as by-products. The residues moreover contain solids from the direct synthesis, which, as fines, are not retained even by cyclones and filters. The solids are composed of silicon, metal chlorides, e.g. AlCl3, metal silicides and soot.
The oligosilanes, in particular the disilanes of the general formula RcCl6-cSi2, in which c is from 0 to 6, make up the predominant part of these residues. However, silane compounds having more than 2 Si—Si bonds, e.g. trisilanes, may also be present.
U.S. Pat. No. 2,681,355 describes a continuous process in which residues with boiling points above 70° C. from direct synthesis of methylchlorosilanes are reacted with hydrogen chloride at temperatures of from 400 to 900° C. in a tube without internals, giving monomeric silanes. The process proceeds without catalysis and is purely thermal. High proportions of by-products, e.g. carbosilanes and polymers, are formed during this reaction. Solids are also produced via carbonization of the residues on the hot reactor walls.
Published U.S. application 2002/0183537 describes a process for the thermal cleavage of the residues of direct synthesis of alkylchlorosilanes in which the required hydrogen chloride is prepared in the same step of the process from H2 and Cl2. The reactor is of complex structure, and the process is therefore difficult to control. Local overheating can easily occur, promoting the formation of by-products. Chlorinated hydrocarbons and chlorinated alkylchlorosilanes can also be produced.